Anodizing is a method of providing an anodic oxide layer or coating on a metal substrate, often used in industry to provide a protective and sometimes cosmetically appealing coating to metal parts. During an anodizing process, a portion of the metal substrate is converted to a metal oxide, thereby forming the anodic oxide layer or anodic oxide coating. The nature of the anodic oxide coatings can depend on a number of factors, including chemical makeup of the metal substrates and the process parameters used in the anodizing processes. In some applications, an anodic oxide coating is colored by infusing one or more dyes within pores of the anodic oxide coating, giving the metal part an attractive colored finish.
Unfortunately, in some cases where certain metal alloy substrates are used, the anodic oxide coating can peel, chip or otherwise delaminate from their metal substrates when exposed to scratching or scraping forces during normal use of the part, or even during certain manufacturing operations such as drilling or machining which might be performed after anodizing. This delamination can cause the underlying metal substrate to be exposed at chipped or peeled regions of the anodic oxide coating, leaving visible chip marks and rendering the metal substrate more susceptible to corrosion. This delamination can be at least partially attributed to alloying elements within the metal substrate that become enriched at an interface between the metal substrate and the anodic oxide coating.
In addition to making the anodic oxide coating more susceptible to delamination, the interfacial enrichment of alloying elements can contribute to the discoloration of the anodic oxide coating, which can detract from the aesthetic appeal of the part. In addition, anodizing metal alloy substrates that are hard tempered can result in the formation of very small groove defects that are detrimental to the function and cosmetics of the anodic oxide coating.